Organic Light emitting display (OLED) devices can be classified as either organic small molecule type or polymeric type based on the nature of electroluminescent material used to create the device. In simplest form, an OLED device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. A simple OLED device can be constructed from an electroluminescent organic small molecule or polymeric layer sandwiched between an electron injection cathode and a hole injection anode. More complicated devices utilize electron and hole transport layers between the above mentioned electrode and the electroluminescent layers.
Common OLED devices include very simple structures comprising a single anode and cathode to more complex devices, such as passive and active matrix displays. A passive matrix display is comprised of orthogonal arrays of anodes and cathodes to form pixels at their intersections, wherein each pixel acts as an OLED device that can be electrically activated independently of other pixels. In active-matrix displays, an array of OLED devices (pixels) are formed in contact with thin film transistors (TFTs) such that each pixel is activated and controlled independently by these TFTs. Full color displays can be constructed using either active or passive matrix designs, as is well known in the art. See for example U.S. Pat. Nos. 5,684,365, 5,294,870, and U.S. Pat. No. 5,294,869.
In prior art multicolor fabrication systems, a masking operation is often first performed to protect the areas that are not to receive an electroluminescent material. The electroluminescent material is then deposited using one of several techniques such as vacuum deposition, and casting and spin coating. The vacuum deposition process generally involves the evaporation of electroluminescent material by heating or by ion bombardment followed by deposition on the substrate by condensation or by a chemical reaction. An inherent limitation of the vacuum deposition process is that the electroluminescent material has to be thermally stable or has to have a thermally stable precursor that can generate the desired material on the substrate by a chemical reaction. This limits the choice of electroluminescent materials that can be used to create display devices.
After deposition of the electroluminescent material, the mask is removed and the mask for the next material layer is placed and the material is deposited. Such techniques are well known in art as shadow masking techniques. Each masking operation increases the cost of fabricating display devices and decreases the device yield. Hence it is advantageous to use methods that do not involve masking.
In U.S. Pat. No. 5,972,419, Roitman et al. disclose an inkjet printing method for making a polymer based electroluminescent device. The electroluminescent polymer containing dopants to produce one of three red, green and blue pixel is dissolved in xylene to form ink. The ink is then dispensed through an inkjet print head at desired locations to make the device. A major limitation of this technology is that the non-aqueous liquids/solvents used to formulate the ink can be hazardous to health and the disposal of which can be prohibitively expensive.
To eliminate the need for potentially harmful solvents, it is possible to use environmental and health-benign supercritical or dense-phase fluids such as carbon dioxide as solvents. Technologies that use supercritical fluid solvents to create thin films are also known. For example, R. D. Smith in U.S. Pat. No. 4,734,227, discloses a method of depositing solid films or creating fine powders through the dissolution of a solid material into a supercritical fluid solution and then rapidly expanding the solution to create particles of the marking material in the form of fine powders or long thin fibers, which may be used to make films. There is a problem with this method in that the free-jet expansion of the supercritical fluid solution results in a non-collimated/defocused spray that cannot be used to create high-resolution patterns directly on a receiver. Further, defocusing leads to losses of the marking material.
Other technologies that deposit a material onto a receiver using gaseous propellants are known. For example, Peeters et al., in U.S. Pat. No. 6,116,718, discloses a print head for use in a marking apparatus in which a propellant gas is passed through a channel, the marking material is introduced controllably into the propellant stream to form a ballistic aerosol for propelling non-colloidal, solid or semi-solid particulate or a liquid, toward a receiver with sufficient kinetic energy to fuse the marking material to the receiver. There is a problem with this technology in that the marking material and propellant stream are two different entities and the propellant is used to impart kinetic energy to the marking material. When the marking material is added into the propellant stream in the channel, a non-colloidal ballistic aerosol is formed prior to exiting the print head. This non-colloidal ballistic aerosol, which is a combination of the marking material and the propellant, is not thermodynamically stable/metastable. As such, the marking material is prone to settling in the propellant stream which, in turn, can cause marking material agglomeration, leading to discharge device obstruction and poor control over marking material deposition.
Huck et al., in WO 02/45868 A2, disclose a method of creating a pattern on a surface of a wafer using compressed carbon dioxide. The method includes dissolving or suspending a material in a solvent phase containing compressed carbon dioxide, and depositing the solution or suspension onto the surface of the wafer, the evaporation of the solvent phase leaving a patterned deposit of the material. The wafer is prepatterned using lithography to provide the wafer with hydrophilic and hydrophobic areas. After deposition of the solution (or suspension) onto the wafer surface followed by the evaporation of the solvent phase, the material (a polymer) sticks to one of the hydrophobic and hydrophilic areas. The solution (or suspension) is deposited on the wafer surface either in the form of liquid drops or a feathered spray.
This method is disadvantaged because deposition using a feathered spray requires that the wafer surface be prepatterned prior to deposition. Hence, direct patterning of the wafer surface is not possible because of the diverging profile (feathered) of the spray. Additionally, a wafer surface that has not been prepatterned can not be patterned using this method. This method also requires time for drying so that the solvent phase of the liquid drops (or feathered spray) can evaporate. During the time associated with solvent phase evaporation, the solvent and the material can diffuse (for example, into the surface or along the surface) degrading the desired pattern.
As such, there is a need for a technology that permits delivery of electroluminescent material to a receiver that reduces the need for post-deposition drying of the receiver.